Optical glass containing Bismuth Oxide

ABSTRACT

Optical glass having a refractive index (n d ) of 1.85 or greater, and an Abbe number (ν d ) falling within the range of 10 to 30, which is suited for molding by precision mold press is provided. The optical glass is characterized by including B 2 O 3 +SiO 2  in an amount of 3 to 60%, Bi 2 O 3  in an amount of 25 to 80%, RO+Rn 2 O in an amount of 5 to 60% (wherein R represents one or more selected from a group consisting of Zn, Ba, Sr, Ca, and Mg; and Rn represents one or more selected from a group consisting of Li, Na, K, and Cs), with each component in the range expressed in oxide-based mole percentage, and is characterized in that transparency in the visible region is high, and that the transition point (Tg) is 480° C. or lower. The optical glass is characterized by having a spectral transmittance of 70% or greater at a wavelength of 600 nm for a thickness of 10 mm.

TECHNICAL FIELD

The present invention relates to an optical glass having high transparency in visible regions, and having optical constants such that the refractive index (n_(d)) is 1.85 or greater, and an Abbe number (ν_(d)) is within a range of 10 to 30, so that the glass is suited for molding by precision mold press.

BACKGROUND ART

Conventionally, optical glass having a high refractive index, and with a high dispersion region are typically of a composition containing a large amount of lead oxide. Because of favorable glass stability and a low glass transition point (Tg), they have been used for precision mold press molding. For example, Patent Document 1 discloses an optical glass for a precision mold press which contains a large amount of lead oxide.

However, a reduction condition is maintained in order to prevent oxidation of the mold when molding by a precision mold press is performed. Therefore, when the glass component contains lead oxide, lead reduced from the glass surface break off and adheres to the mold surface. Accordingly, there have been problems in that the precision face of the mold is not maintained. Also, lead oxide is toxic to the environment so that glass free from lead oxide has been desired.

To meet such needs, many types of optical glass for press molding, having a high refractive index, with a high dispersion region, and not containing lead oxide, have been developed. However, most of those are of a phosphate-based system. For example, Patent Document 2 and Patent Document 3 disclose P₂O₅—Nb₂O₅—WO₃—(K₂O, Na₂O, Li₂O) based glass, and Patent Document 4 discloses P₂O₅—Nb₂O₅—TiO₂—Bi₂O₃—Na₂O based glass. However, many of these types of glass have a Tg of higher than 480° C., although such Tgs may be referred to as being low. Furthermore, a large amount of Nb₂O₅ must be introduced for achieving a high refractive index and high dispersion; therefore, there may be disadvantages in that devitrification resistance of the glass is not so high. Moreover, an optical glass having optical constants such that its refractive index is 1.95 or greater, and its dispersion is 20 or less has not been obtained from the phosphoric acid-based glass.

Furthermore, compositions including a large amount of Bi₂O₃ are well known for glasses with low Tg. For example, Nonpatent Documents 1, 2, 3, and 4 disclose Bi₂O₃—Ga₂O₃—PbO based glass, Bi₂O₃—Ga₂O₃-(Li₂O, K₂O, Cs₂O) based glass, and Bi₂O₃—GeO₂ based glass. Although these types of glass have a Tg of 480° C. or lower, an absorption edge of the glass is more than 450 nm. Hence, transparency in the visible region may be drastically lost, and thus, these types of glass cannot be used as an optical lens required to have high transparency in the visible region.

[Patent Document 1] Japanese Patent Application Laid Open No. H1-308843

[Patent Document 2] Japanese Patent Application Laid Open No. 2003-321245

[Patent Document 3] Japanese Patent Application Laid Open No. H8-157231

[Patent Document 4] Japanese Patent Application Laid Open No. 2003-300751

[Nonpatent Document 1] Physics and Chemistry of Glasses, p 119, Vol. 27, No. 3, June 1986

[Nonpatent Document 2] Glass Technology, p 106, Vol. 28, No. 2, April 1987

[Nonpatent Document 3] American Ceramic Society Bulletin, p 1543, Vol. 71, No. 10, October 1992

[Nonpatent Document 4] American Ceramic Society, p 1017, Vol. 77, No. 4, October 1994

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

An object of the present invention is to provide a novel optical glass having a refractive index (n_(d)) of 1.85 or greater, an Abbe number (ν_(d)) falling within the range of 10 to 30, and having high transparency in the visible region, and a glass transition point (Tg) of 480° C. or lower, which is suited for molding by a precision mold press.

Means for Solving the Problems

The present inventor carried out considerable research in order to solve the aforementioned problems, as a result of which he found that, through incorporating a large amount of Bi₂O₃ in a system that is completely different from the preexisting phosphoric acid salt system, transparency that is satisfactory for optical lenses in the visible region, high refractive index (n_(d)=1.85 or greater), and high Abbe number (ν_(d)=10 to 30) could be achieved while keeping the glass transition point (Tg) of 480° C. or lower, and further found that these types of glass are extremely favorable for precision mold presses, resulting in the present invention.

More specifically, in a first aspect of the present invention, an optical glass includes Bi₂O₃ in an amount of 25 to 80% expressed in oxide-based mole percentage, and having a refractive index (n_(d)) of 1.85 or greater, and an Abbe number (ν_(d)) of 10 to 30.

In a second aspect of the present invention, the optical glass according to the first aspect has a spectral transmittance of 70% or greater at a wavelength of 600 nm, for a thickness of 10 mm (optical path length: 10 mm).

In a third aspect of the present invention, the optical glass according to the first or second aspect has a transition point (Tg) of 480° C. or lower.

In a fourth aspect of the present invention, an optical glass includes B₂O₃+SiO₂ in an amount of 3 to 60%, and/or Bi₂O₃ in an amount of 25 to 80%, and/or RO+Rn₂O in an amount of 5 to 60% (wherein R represents one or more selected from a group consisting of Zn, Ba, Sr, Ca, and Mg; and Rn represents one or more selected from a group consisting of Li, Na, K, and Cs), and/or Sb₂O₃+As₂O₃ in an amount of 0 to 5%, with each component in the range expressed in oxide-based mole percentage, wherein the wavelength at which the spectral transmittance of 70% is exhibited is 550 nm or less; the refractive index (n_(d)) is 1.85 or greater; and the Abbe number (ν_(d)) is 10 to 30.

In a fifth aspect of the present invention, the optical glass according to any one of the first to fourth aspects has a part or all of B₂O₃, and/or SiO₂ substituted with GeO₂.

In a sixth aspect of the present invention, the optical glass according to any one of the first to fifth aspects includes one or two of Al₂O₃, and/or Ga₂O₃ components in an amount of 0 to 20% expressed in oxide-based mole percentage.

In a seventh aspect of the present invention, the optical glass according to any one of the first to sixth aspects includes P₂O₅ in an amount of 0 to 8% expressed in oxide-based mole percentage.

In an eighth aspect of the present invention, the optical glass according to any one of the first to seventh aspects includes TiO₂ in an amount of 0 to 20% expressed in oxide-based mole percentage.

In a ninth aspect of the present invention, the optical glass according to any one of the first to eighth aspects includes one or more of La₂O₃, and/or Y₂O₃, and/or Gd₂O₃ components in an amount of 0 to 15% expressed in oxide-based mole percentage.

In a tenth aspect of the present invention, the optical glass according to any one of the first to ninth aspects includes one or more of ZrO₂, and/or SnO₂, and/or Nb₂O₅, and/or Ta₂O₅, and/or WO₃ components in an amount of 0 to 10% expressed in mol % based on oxide.

In an eleventh aspect of the present invention, the optical glass according to any one of the first to tenth aspects has an absorption edge of 450 nm or lower.

In a twelfth aspect of the present invention, the optical glass according to any one of the first to eleventh aspects has a B₂O₃/SiO₂ value (mol % ratio), expressed in oxide-based mole percentage, is 0.2 to 5.

In a thirteenth aspect of the present invention, the optical glass according to any one of the first to twelfth aspects has a total content of Li₂O, Na₂O, and K₂O exceeding 8% expressed in oxide-based mole percentage.

In a fourteenth aspect of the present invention, the optical glass according to any one of the first to thirteenth aspects has a total content of La₂O₃, ZrO₂, TiO₂, SrO, Na₂O, and Li₂O exceeding 10% expressed in oxide-based mole percentage.

In a fifteenth aspect of the present invention, the optical glass according to any one of the first to fourteenth aspects is for precision molding.

In a sixteenth aspect of the present invention, an optical element is obtained by molding the glass for precision molding according to the fifteenth aspect.

EFFECTS OF THE INVENTION

The optical glass of the present invention includes Bi₂O₃ in a large amount as a glass component; therefore, transparency which is satisfactory for optical lenses in the visible region, a high refractive index (n_(d)=1.85 or greater) and a low Abbe number (ν_(d)=10 to 30) can be achieved while keeping the glass transition point (Tg) at 480° C. or lower. Accordingly, an optical glass suited for molding by a precision mold press can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a spectral transmittance curve for the glass of Embodiments 1 and 17. The abscissa represents the wavelength (nm), and the ordinate represents the spectral transmittance (%).

PREFERRED MODE FOR CARRYING OUT THE INVENTION

Grounds for defining the composition range of each component that constitutes the optical glass of the present invention as described above will be described below. Each component is expressed in oxide-based mole percentage.

B₂O₃ and SiO₂ components are oxides for forming glass, either of which is essential for obtaining stable glass. In order to obtain stable glass, the lower total content limit of one or two of these components is preferably 3%, more preferably 5%, and most preferably 10%. However, for attaining a refractive index of 1.85 or greater and a Tg of 480° C. or lower, the upper content limit is preferably 60%, more preferably 55%, and most preferably 50%. The object of the present invention can be achieved even if these two components are introduced into the glass alone; however, concurrent use is preferred because melting properties, stability and chemical durability of the glass may be enhanced, accompanied by improvement of transparency in the visible region, when they are used concurrently. Additionally, in order to maximize the aforementioned effect, mole percentage ratio of B₂O₃/SiO₂ desirably falls within the range of 0.2 to 5.

The GeO₂ component is effective in improving the stability and refractive index of the glass, and further, is responsible for high dispersion; therefore, it is an optional component which can be introduced into the glass to substitute for a part or all of B₂O₃ or SiO₂. However, due to its expensiveness, the upper content limit is preferably 50%, more preferably 45%, and most preferably 35% in order to keeping the Tg at 480° C. or lower.

The Bi₂O₃ component contributes greatly towards improving the stability of the glass, and is necessary for achieving the object of the present invention which is, in particular, to attain a refractive index (n_(d)) of 1.85 or greater and a Tg of 480° C. or lower. Because the refractive index and the dispersion in the present invention are strongly dependent on the content of Bi₂O₃, a content that is too low may not achieve the desired high refractive index and high dispersion. However, when the content is too high, stability of the glass is significantly lowered. Therefore, the content preferably falls within the range of 25 to 80%. A more preferable range is 25% to 70%, and a most preferable range is 25% to 60%.

The RO, Rn₂O (wherein R represents one or more selected from a group consisting of Zn, Ba, Sr, Ca, and Mg; and Rn represents one or more selected from a group consisting of Li, Na, K, and Cs) components are effective in improving the melting property and stability of the glass, and in lowering the Tg. Furthermore, they play a major role in improving the transparency of the glass in the visible region; therefore, either one of these components is essential. When total content of one or two of these components (RO+Rn₂O) is too low, the effect is not realized, while too high a content may result in deterioration of the stability. Hence, the total content of these components is preferably from 5 to 60%. The total content falls more preferably within the range of 8 to 55%, and particularly preferably within the range of 15 to 50%. However, when RO is introduced alone, suitable content for achieving the aforementioned effect falls within the range of 5 to 50%, more preferably within the range of 10 to 40%, and most preferably within the range of 15 to 40%. Among the Rn₂O components, BaO and ZnO components are the most effective; therefore, either of these is preferably included. Additionally, when one or two among SrO, CaO, and MgO is concurrently included, stability, chemical durability, and transmittance in the visible region of the glass are further improved. Therefore, it is more preferred that one or two of these components, and either one or both of BaO and ZnO be concurrently included. Moreover, when Rn₂O is introduced alone, suitable content for achieving the aforementioned effect falls preferably within the range of 5 to 45%, more preferably within the range of 8 to 40%, and most preferably within the range of 10 to 40%. Of the RO components, Li₂O and Na₂O components best exhibit the aforementioned effect; therefore, it is preferred that either one or both of these be included. However, in order to particularly improve the chemical durability of the glass, use in combination with K₂O is preferred. It is preferred that total content of Li₂O, Na₂O, and K₂O components exceed 8%. The total content is more preferably 8.5% or greater, and particularly preferably 9% or greater.

Furthermore, in order to realize the aforementioned effect more efficaciously, the total content of La₂O₃, ZrO₂, TiO₂, SrO, Na₂O, and Li₂O may be a predetermined value or greater expressed in oxide-based mole percentage, and thus, a synergistic action with the aforementioned effect can be expected. Therefore, the total content of La₂O₃, ZrO₂, TiO₂, SrO, Na₂O, and Li₂O preferably exceeds 10%. The total content is more preferably 10.5% or greater, and particularly preferably 11% or greater.

The Al₂O₃, Ga₂O₃ components can be optionally added because they are effective in improving melting properties and chemical durability of the glass. It is particularly desired that they be introduced to substitute for B₂O₃ or SiO₂ or GeO₂. However, in the case of the composition containing B₂O₃ or SiO₂ or GeO₂ at a content exceeding 40%, introduction of these components may lead to the Tg exceeding 480° C. Therefore, these components should be introduced to a composition with the content of B₂O₃ or SiO₂ or GeO₂ being 40% or less, more preferably 35% or less, and particularly preferably 30% or less. When the total content of one or two of these components is low, the effect may not be exhibited, while too great a total content may result in deterioration of the melting properties and stability of the glass, and the Tg may increase to a great extent. Therefore, the total content of Al₂O₃ and Ga₂O₃ preferably falls within the range of 0 to 20%. The total content falls more preferably within the range of 0.1 to 20%, even more preferably within the range of 0.5 to 10%, and particularly preferably within the range of 0.5 to 5%.

The P₂O₅ component can be optionally added because it is effective in improving the melting property of the glass. When the amount is too small, the effect may not be exhibited, while too great an amount of the same may deteriorate the melting property of the glass. Therefore, the content of this component preferably falls within the range of 0 to 8%. The content falls more preferably within the range of 0.1 to 8%, even more preferably within the range of 0.5 to 5%, and particularly preferably within the range of 0.5 to 4%.

The TiO₂ component can be optionally added because it is effective contributing to improvement of the refractive index and chemical durability of the glass, and to attaining high dispersion. When the content is too low, the effect may not be exhibited, while too great a total content may deteriorate the melting property of the glass and the stability of the glass, and the Tg may increase to a great extent. Therefore, the content of this component preferably falls within the range of 0 to 20%. The content falls more preferably within the range of 0.1 to 20%, even more preferably within the range of 0.5 to 18%, and particularly preferably within the range of 0.5 to 15%.

The La₂O₃, Y₂O₃, Gd₂O₃ components can be optionally added because they are greatly effective in improving the refractive index, chemical durability and transparency of the glass, and in adjusting the dispersion. When the total content of one or two or more of these components is too low, the effect may not be exhibited, while too great a total content may result in not only deterioration of the melting property and stability of the glass, but also elevation of the Tg. Therefore, the content preferably falls within the range of 0 to 15%. The content falls more preferably within the range of 0.1 to 15%, even more preferably within the range of 0.5 to 15%, and particularly preferably within the range of 0.5 to 10%.

The ZrO₂, SnO₂, Nb₂O₅, Ta₂O₅, WO₃ components can be optionally added because they are effective in improving the refractive index and chemical durability of the glass. When the total content of one or two or more of these components is too low, the effect may not be exhibited, while too great a total content may result in deterioration of the melting properties and stability of the glass, and the Tg may increase to a great extent. Therefore, the content preferably falls within the range of 0 to 10%. The content falls more preferably within the range of 0.1 to 10%, even more preferably within the range of 0.5 to 8%, and particularly preferably within the range of 0.5 to 5%.

The Sb₂O₃ or As₂O₃ component can be added for defoaming the glass during melting, but the amount of up to 5% is sufficient.

It is preferred that PbO which is a component that is inappropriate for optical glasses for a mold press be not included.

The optical glass of the present invention has a refractive index (n_(d)) of 1.85 or greater, and an Abbe number (ν_(d)) falling within the range of 10 to 30. More preferred ranges of n_(d) and ν_(d) are 1.90 or greater and 10 to 25, respectively, and most preferred ranges are 1.92 or greater and 10 to 25, respectively.

In addition to high refractive index and high dispersion, the optical glass of the present invention has a transition point (Tg) of 480° C. or lower. A more preferred range of the Tg is 350 to 480° C., and the most preferred range is 360 to 460° C.

Measurement of transmittance herein was carried out in accordance with Japan Optical Glass Industrial Association Standards, JOGIS02-1975. With respect to the transparency of the optical glass of the present invention as represented by transmittance of the glass, the wavelength at which the spectral transmittance of 70% is exhibited, using a sample having a thickness of 10 mm may be 600 nm or lower, more preferably 550 nm or lower, and most preferably 530 nm or lower.

EMBODIMENTS

The optical glass of the present invention can be manufactured by the following method. More specifically, each starting material (oxide, carbonic acid salt, nitric acid salt, phosphoric acid salt, sulfuric acid salt or the like) was weighed in a given amount. After mixing homogeneously, the mixture was placed in a quartz crucible, an alumina crucible, a gold crucible, a platinum crucible, a gold or platinum alloy crucible, an iridium alloy crucible or the like, and was melted in a melting furnace at 800 to 1250° C. for 2 to 10 hrs while stirring to permit homogenization. Thereafter, the temperature was lowered to a suitable point, and casting in a mold or the like was performed to obtain a glass.

The present invention will be explained below by way of Embodiments; however, the present invention is not limited to these Embodiments.

The materials were weighed such that types of glass of 400 g having the predetermined composition shown in Tables 1 to 4 were provided. After mixing homogeneously, the mixture was melted using quartz and a platinum crucible at 900 to 1100° C. for 2 to 3 hrs. Thereafter, the temperature was lowered to 750 to 900° C., and further incubated for 40 min. The mixture was cast in a mold to manufacture glass. Characteristics of the resulting glass are shown in Tables 1 to 4. Furthermore, spectral transmittance was measured on Embodiment 1 and Embodiment 17. The results are shown in FIG. 1.

Measurement of the transmittance was carried out in accordance with Japan Optical Glass Industrial Association Standards JOGIS02. In the present invention, the transmittance, not the degree of coloring, is presented. Specifically, spectral transmittance at 200 to 800 nm on the parallel opposing face-polished product having a thickness of 10±0.1 mm was measured in accordance with JIS Z8722. The value of (wavelength at which the transmittance is 70%)/(wavelength at which the transmittance is 5%) is shown, which was determined by rounding off the values.

The transition point (Tg) was measured with a thermodilatometer at a temperature-ascending rate of 4° C./min.

For the refractive index (n_(d)) and the Abbe number (ν_(d)), the glass was kept at around the transition point (Tg) for 2 hrs, then cooled at a slow temperature-descending rate of—25° C./Hr, and measured according to JOGIS01-2003.

Additionally, in a similar manner to the aforementioned Embodiments, a Comparative Example having the composition of 50 B₂O₃-20 SiO₂-30 Bi₂O₃ (in mole percentage) as shown in Table 4 was manufactured. The glass almost completely devitrified, and a sample which could be used for evaluation of the physical properties could not be obtained.

TABLE 1 Embodiments 1 2 3 4 5 6 7 8 9 10 B₂O₃ 26 10 14 10 10 20 26 26 SiO₂ 10 20 20 20 15 30 10 10 10 GeO₂ 5 30 P₂O₅ Bi₂O₃ 30 30 26 30 30 30 30 30 30 30 Al₂O₃ 4 Ga₂O₃ 4 ZnO 10 10 10 10 10 10 10 10 10 10 BaO 10 10 10 10 10 10 10 10 10 10 SrO CaO 10 10 8 10 10 10 MgO 2 K₂O Na₂O Li₂O 10 10 10 10 10 10 10 10 10 10 TiO₂ 10 ZrO₂ La₂O₃ 4 Gd₂O₃ Y₂O₃ SnO₂ WO₃ Nb₂O₅ Ta₂O₅ Sb₂O₃ 0.05 0.05 As₂O₃ 0.05 Tg 415 405 405 408 406 410 423 413 412 410 n_(d) 1.968 1.990 1.953 1.988 1.993 2.018 2.038 2.029 1.946 1.948 ν_(d) 21.6 19.7 21.1 19.8 19.7 18.3 18.2 18.9 21.9 21.8 λ_(70%) 470 500 470 500 500 525 515 495 470 470 λ_(5%) 410 420 410 420 420 440 430 420 410 410

TABLE 2 Embodiments 11 12 13 14 15 16 17 18 19 20 B₂O₃ 26 30 25 25 25 25 25 25 25 25 SiO₂ 10 20 15 14 15 15 15 15 15 15 GeO₂ P₂O₅ 1 Bi₂O₃ 30 30 30 30 30 35 35 35 35 35 Al₂O₃ Ga₂O₃ ZnO 10 10 20 20 20 10 5 BaO 10 10 9.5 10 5 10 10 10 10 SrO 5 5 CaO MgO K₂O 10 Na₂O 10 Li₂O 10 10 10 10 12 TiO₂ ZrO₂ La₂O₃ Gd₂O₃ 3 Y₂O₃ 4 SnO₂ 0.5 WO₃ Nb₂O₅ Ta₂O₅ Sb₂O₃ 0.02 As₂O₃ 0.05 Tg 418 438 445 440 408 390 395 390 395 394 n_(d) 1.965 1.964 1.949 1.947 1.972 2.018 2.002 1.977 1.957 2.011 ν_(d) 21.7 21.5 22.0 22.1 21.1 19.4 19.8 19.5 19.4 19.7 λ_(70%) 470 470 470 470 470 500 490 490 490 500 λ_(5%) 410 405 405 405 405 415 415 410 410 420

TABLE 3 Embodiments 21 22 23 24 25 26 27 28 29 30 B₂O₃ 25 25 25 25 5 30 10 25 25 28 SiO₂ 15 15 15 15 15 10 15 15 10 GeO₂ P₂O₅ Bi₂O₃ 35 34 34 34 40 40 40 40 40 42 Al₂O₃ Ga₂O₃ ZnO 5 5 5 10 10 10 10 6 BaO 10 10 10 10 10 10 10 10 SrO 2 CaO 10 10 MgO K₂O Na₂O Li₂O 12 10 10 10 10 10 10 10 10 10 TiO₂ 2 ZrO₂ 3 La₂O₃ Gd₂O₃ Y₂O₃ SnO₂ WO₃ 1 Nb₂O₅ 1 Ta₂O₅ 1 Sb₂O₃ As₂O₃ 0.02 Tg 396 398 398 400 400 395 395 388 392 365 n_(d) 2.008 1.994 2.003 1.997 2.059 2.005 2.037 2.062 2.032 2.11076 ν_(d) 19.6 20.4 19.8 20.6 17.5 19.8 18.3 18.3 18.9 16.9 λ_(70%) 500 490 490 490 530 490 520 500 500 480 λ_(5%) 420 415 415 415 430 410 420 410 415 425

TABLE 4 Embodiments Comparative 31 32 33 34 35 Example B₂O₃ 29.93 27 25 26 25 50 SiO₂ 9.97 10 10 15 15 20 GeO₂ P₂O₅ Bi₂O₃ 34.91 43 40 34 35 30 Al₂O₃ Ga₂O₃ ZnO 4.99 6.50 10 10 7 BaO 6.98 1 3 SrO 2.99 2.5 CaO MgO K₂O Na₂O Li₂O 9.98 10 8 10 10 TiO₂ ZrO₂ 2 3 2 La₂O₃ 2 3 3 Gd₂O₃ 3 Y₂O₃ SnO₂ WO₃ Nb₂O₅ Ta₂O₅ Sb₂O₃ 0.25 As₂O₃ Tg 386 360 393 400 400 Devitrification n_(d) 2.014 2.101 2.096 2.036 20.44 ν_(d) 19.4 17.1 18.1 19.6 19.2 λ_(70%) 460 482 470 470 470 λ_(5%) 417 425 415 415 415

From Tables 1 to 4, it was elucidated that all glass of Embodiments had a n_(d) of 1.90 or greater, a ν_(d) falling within the range of 10 to 25, and a Tg of 450° C. or lower. Moreover, from the spectral transmittance curve shown in FIG. 1, it is understood that the glass of the present invention has high transparency. Because the absorption edge of the glass shifts toward the lower wavelength as the thickness of the glass becomes smaller, and the transparency at the short wavelength varies depending on the thickness, in the present invention, values were obtained for wavelength at which spectral transmittances of 70% and 5% (λ₇₀% and λ₅%) are exhibited, for a thickness of 10 mm in the present invention, and transparency of the glass was evaluated. The results are shown in Tables 1 to 3. Herein, the wavelength at which the spectral transmittance of 5% is exhibited is referred to as glass absorption edge. It was demonstrated that all glass had wavelengths that exhibit the spectral transmittance of 70%, of being 600 nm or lower, and absorption edges of 450 nm or lower, suggesting high transparency in the visible region.

Additionally, as a result of experiments with a precision mold press with these types of glass, lenses with high precision were obtained, and favorable transferability property was exhibited without adhesion of the glass to the mold.

INDUSTRIAL APPLICABILITY

As described above, the optical glass of the present invention has as an optical constant, a refractive index (n_(d)) of 1.94 or greater, and has high transparency in the visible region, having a transition point (Tg) of 480° C. or lower, and is suited for molding by a precision mold press. In addition, the optical glass can be applied to any of: a method in which a melted glass is directly molded to obtain an optical element such as a lens; and a method in which molding is performed from a melted glass via a preform mold (which can be obtained by a method of molding through obtaining a melted glass with a mold, a method of press molding, or a method of grinding, polishing) to obtain an optical element such as a lens.

Moreover, the optical glass of the present invention is suitable for lenses for optical communication, the demand for which has been rapidly increasing in recent years. A lens for optical communication is a glass lens which functions by injecting a laser beam emitted from a luminous body such as semiconductor laser into an optical fiber at a high efficiency, and is a micro-optical accessory that is necessary for members for optical communication. Although a ball lens, a nonspherical lens or the like may be used as this lens, a required characteristic therefore is a high refractive index. In particular, the optical glass of the present invention is suited for molding by precision mold press in cases in which it is used as a nonspherical lens. 

1. An optical glass comprising Bi₂O₃ in an amount of 25 to 80% expressed in oxide-based mole percentage, having a refractive index (n_(d)) of 1.85 or greater, and an Abbe number (ν_(d)) of 10 to
 30. 2. An optical glass according to claim 1, having a spectral transmittance of 70% or greater at a wavelength of 600 nm, for a thickness of 10 mm.
 3. An optical glass according to claim 1, having a transition point (Tg) of 480° C. or lower.
 4. An optical glass comprising B₂O₃+SiO₂ in an amount of 3 to 60%, and/or Bi₂O₃ in an amount of 25 to 80%, and/or RO+Rn₂O in an amount of 5 to 60% (wherein R represents one or more selected from a group consisting of Zn, Ba, Sr, Ca, and Mg; and Rn represents one or more selected from a group consisting of Li, Na, K, and Cs), and/or Sb₂O₃+As₂O₃ in an amount of 0 to 5%, with each component in the range expressed in oxide-based mole percentage, wherein a wavelength at which a spectral transmittance of 70% is exhibited is 550 nm or lower; the refractive index (n_(d)) is 1.85 or greater; and the Abbe number (ν_(d)) is 10 to
 30. 5. An optical glass according to claim 4 wherein a part or all of B₂O₃ and/or SiO₂ is substituted with GeO₂.
 6. An optical glass according to claim 1 comprising one or two of Al₂O₃ and/or Ga₂O₃ components in an amount of 0 to 20% expressed in oxide-based mole percentage.
 7. An optical glass according to claim 1 comprising P₂O₅ in an amount of 0 to 8% expressed in oxide-based mole percentage.
 8. An optical glass according to claim 1 comprising TiO₂ in an amount of 0 to 20% expressed in oxide-based mole percentage.
 9. An optical glass according to claim 1 comprising one or more of La₂O₃, and/or Y₂O₃, and/or Gd₂O₃ components in an amount of 0 to 15% expressed in oxide-based mole percentage.
 10. An optical glass according to claim 1 which comprises one or more of ZrO₂, and/or SnO₂, and/or Nb₂O₅, and/or Ta₂O₅, and/or WO₃ components in an amount of 0 to 10% expressed in oxide-based mole percentage.
 11. An optical glass according to claim 1 wherein an absorption edge is 450 nm or lower.
 12. An optical glass according to claim 4 wherein the B₂O₃/SiO₂ value (mole percentage ratio), expressed in oxide-based mole percentage, is 0.2 to
 5. 13. An optical glass according to claim 4 wherein total content of Li₂O, Na₂O, and K₂O exceeds 8% expressed in oxide-based mole percentage.
 14. An optical glass according to claim 1 wherein total content of La₂O₃, ZrO₂, TiO₂, SrO, Na₂O, and Li₂O exceeds 10% expressed in oxide-based mole percentage.
 15. An optical glass for precision molding, made of an optical glass according to claim
 1. 16. An optical element obtained by molding the glass for precision molding according to claim
 15. 